Electronic circuit



May 13, 1941. w. H. Buss ELECTRONIC. CIRCUIT Filed Aug. 51, 1938 INVENTOR NPPf/V 64 5J' ATTORNEY Patented May 13, 1941 UNITED stares rter Fries `2,124.1,533 ELECTRONIC ontcurr Warren H. Bliss, Brooklyn, N. Y., assigner to Radio Corporation of America, a corporation of.

Delaware Application Laugust 31, 1938, Serial No. 227,643

t?. Claims. (Cl. 179-1715) This invention relates to a circuit which nds particular application in connection with facsimile transmission systems and which is also applicable to use in measuring systems or the like.

In its essential portions the present invention, as applied to a system for transmitting electrical signals which represent the varying light values of a subject as used in the transmission of pictures, it is frequently desirable, in order to avoid the use of the so-called direct current famplier systems, to provide ways and means for providing a modulated alternating current output from the system. In this form of system the magnitude of the envelope of the modulated alternating current output is instantaneously proportional to the amount of light upon a related elemental area of the subject of which the electro-optical likeness is to be produced at receiver points.

In its essence the present invention embodies the use of a secondary emission type of phototube with a tone carrier frequency applied electrostatically in a cathode circuit of the tube to which tube a counter-electromotive force is also applied for balancing out a capacity current which may result in the absence of illumination. If such capacity currents should be high in value they would preclude the possibility of relatively greater change in light values from being able to cause any substantial depth of modulation. In combination with this arrangement, there has been provided a suitable ampliiier and either a filter arrangement through which the resultant signals may be fed to a transmitter or a sultable rectifier from which the direct current output may be utilized for the purpose of obtaining monitor indications or for the purpose of obtaining direct readings in cases where it is desirable to measure varying light values as in photometric investigations.

In general, many facsimile scanner modulator systems, as now used in the art, have been found to be non-linear over their entire operating range and some systems of the type now known do not produce as great a contrast of ratio output for a change from white to black or vice versa 1n the scanning operation as is desirable to produce good quality pictures. y

Accordingly, as one of tlieprimary objects of this invention, this system has as its object that of overcoming these defects of the prior art.

Still other objects and advantages of the invention are to provide aV circuit arrangement for use infacsimile transmission systems Whlch 1s .t relatively simple in its arrangement and which will, nevertheless, provide a relatively high output signal which will be substantially linear throughout the entire operational range.

Still other objects and advantages of the invention are to provide a circuit arrangement for use in facsimile systems which are eticient in operation, readily set-up, and which embodies a relatively few number of component parts.

Still other and further objects of the invention will become apparent from a description in connection with the accompanying drawing, wherein the single figure thereof illustrates in conventional manner one form of circuit for carrying out the aims and objects of the present. invention.A

In the system, as it is preferably constituted, there is provided a form of secondary emission photo-tube onto which the light of the image, as it is scanned, is projected. This photo-tube has a carrier or tone frequency volt-age applied thereto and, at the same time, there is applied through the same tube a counterelectromotive force which precludes the possibility of any high capacity leakage current flowing in the anode circuit of the tube. The output energy from the secondary emission photo-tube which translates the variable intensities of light and shade on the picture subject into electric currents is then suitably amplied and passed through a suitable form of bandpass lter to modulate or to key a suitable form of transmitter., From the transmittel` the resultant output signal can be directedto any desired form of receiverapparatus by either a radio or a wire-line connecting channel.

Various other objects and advantages of the present invention will become apparent to those skilled in the art from the following description particularly when considered with the drawing wherein the single ligure shows one form of the present invention.

Referring now more particularly to the drawing for a further understanding of the invention, there is, as is known in the prior art, provided in connection with facsimile apparatus, a scanning drum I I which is arranged to rotate, for example, as shown bythe arrow. Supported upon the scanning drum I I is a record or subject of which it is desired to produce at receiving points an electro-optical likeness. Onto the recordcarried upon the drum I I there is projected by way of the optical system I2 a spot of light I3 from the source I4. Intermediate the optical system I2 and the surface of the drum II or at any point intermediate the drum and the light source I4, there may be provided, as is known in the art, any suitable is proportional to the intensity of light and shadow l of the record instantaneously illuminated. This reflected light is then directed by way ofthe optical system I5 so as to energize or activatea suit.- able light translating element. According: to the present invention the light translating element is provided in the form of a secondary emission type photo-tube I'I in which is contained a photoelectric cathode element I8, a screen orY collector electrode-I9 and a dynode or anode 2li. Such a tube may be 'a .tubel of the general type'which is known in the art and has been sold to the trade underthe designated indicia type C-704f In accordance with the amount of light from Vthe record subject which falls upon the photosensitized cathode element I8 will depend the number of photoelectrons released fromthe light sensitive electrode element I8 thereof. As has already'been explained inthe art, it is usually desirable; inV order to avoid the useof the socalled direct current amplifiers; which are morev diflicult to'op'erate than.V the better known A. C. type, to modulate Vdirectly by the'light translating element a suitable tone frequency which may be an alternating current usually of sine wave formation and of 'a frequency of theorder of 3000 cycles to 5000 cycles or higher depending upon the recording speeds desired.

In accordance with-the present invention the tone frequency which is developed 'by any suitable form tone frequency generator (not shown)` is supplied at the terminal points 2I Vand serves to energize the primary windingsY 23 and 2t4 of the transformers 25, 26. The secondary winding 21 of the transformer 25 is connected at one end to a ground or'earth connection point 28 and at the other end through a resistor 23 to the cathode element I8 of the secondary emission photo-tube I'I. The screen orcollector electrode I9 of the secondaryemission photo-tube II is connected'with a Vsource of voltage (for example.' of the order of 250 volts) 'at the terminal point 30'while the dynode or anode 20 of the secondary emission photo-tube Il' is connected .to one end of a resistor 32 and also to one terminal of `a condenser 33, later to be referred to.

"It, of course. is'apparent that with such an arrangement under'conditions when the photocathode I8 of the secondary emissiveV photo-tube I'I is unilluminated, there may result some capacity current flowing through the' tube. It is desirable for the reasons above stated to V'avoid such a current path and, therefore', there is supplied' to thetube by way of the secondary ywinding 3| ofthe transformer 26 a counter-electromotive force derived from the same tone frequency 'or-carrier which is supplied to the cathode element I8 of the secondary emission photo-tube Il. The circuit for supplying .this` counter-electromotive force comprises, in addition to the .seconda'rywindingI `of the transformer 126,-also apluralityof potentiometers 3.5 andtandrheothe transformer 26 is series connected with the two potentiometers 35 and 3S. Of these two potentiometers it is preferable to have one of relatively high resistance so that it may be used for making the coarse adjustments by varying the position of the slider element li@ while the other potentiometer 3&5 may be of relatively low resistance for making more accurate adjustments. The exact amount of resistance should be determined by varying the position of the slider 4I. The slider element 4i! of the potentiometer 35 is connected to one terminal of the condenser 39, the other terminal of which connects to the lower vend' ofthe output resistor 32 of the secondary emission photo-tube II. The slider element 4I of Vthe potentiometer 35 connects to the terminal point 42 to which positive voltage is applied and which voltage may be of the general order indicated by the drawing.

Also, as shown, the fixed terminal of the rheostat 3l is connected to the righthand terminal of the condenser 3S and also to the lower endV of the' resistor 32 while the fixedterminal of the rheostat 38 is connected also to the terminal connection point 42 to which the positive voltage is supplied.

The sliderA elements 3 and Il of the rheostats 3l and 38; respectively, are connected together by wayof the conductor d5. As was the case with the potentiometersl 35, 3G the rheostat 3l is preferably one of high resistance so'as to be used for @coarse adjustment while the rheostat 38 of relatively low resistance is used for makingjrelativelyflne adjustments by varying the position of the slider element M. j

' The amplifier for ampliiying'the output energy from the tube II preferably comprises a plurality of vacuum tube amplifiers fi'and 48 and their associated resistors; vcondensers and transformers. The righthand terminal of the input condenser 33 is connected to the control .electrode @il of the tube 3 Zand also to one end of the leak resistor 5B. Thebtherend of the leak resistor 5I) connects to the ground connection point 2%. The suppressor 'grid 5I`and the cathode 52 of the tube 46 are connected together `in known manner and connect toground 2S through the usual shunt path which comprises'a resistor 53 `and a shunt condenser 5A. The screen grid 55 is connected to the high potential terminal 3B lby way of the resistor 56 whichY is shunted by a condenser 57 having its lower terminal connected to ground at 2B.

' Heater current for heating the cathode 52 is supplied in the usualV manner and is not shown. The anode or plate electrode 53 ofthe tube liialso connects to the high' potential terminal 3G by'wayof the output resistoril and, at the Sametime, is connected 'toone terminal of a blocking condenser 60 which' couples the voutput of thetube e6' into the input of the tube 8. The opposite endof the blocking condenser Si) connects to the controlv electrode 6I of the amplifier tube'fl and also to theupper end of the leak resistor 62 whoseV lower lend is grounded at'28. The cathode element 63 ofthe tube 48 isk connected as usual to ground( 28 through the shunt path comprising the cathode resistorll and a bypass condenser 65. The anode or plate electrode 55 isrconnected through the primary winding 51 of theoutput transformer 68; tothe terminal Vvpoint- 38 n from which the high potential for supplying anode voltage isderived. .Thesecondary winding $9 of the transformeris connected. through; 'switching element *I0V when? the. switch. armszarc' 1on1 toterminalpomtsgi l:,i 'l2 intaabandnass senses ter I3 and from that point to the modulator and transmitter in order to key or to modulate the transmitter equipment for sending signals to points of reception.

The upper cut-off frequency of the bandp-ass filter is preferably adjusted below twice the Value of the tone or carrier frequency supplied to the terminal points 2l. The lower cut-olf frequency of the filter i3 is not critical but should be far enough below the carrier frequency to give a pass band width Wide enough not to increase the build-up time of the signals.

In cases where it is desired to provide direct recordings or to monitor the signal appearing in the output of the amplifier 48 the switch arms 'i3 may be moved to their upper contacts 'I4 to energize a rectiiier element 'I5 from which direct current output may be obtained to provide in any suitable form of indicating device 'I6 which is made responsive to direct currents an indication of the relative intensity of the light falling upon the photosensitive cathode element I8 of the secondary emission photo-tube Il.

While the operation of the system, as above described in its constituent parts, is substantially evident as to the connections, it will be seen that substantial increase in the current ilow in the conventional types of photo-tubes is possible by the increased current ilow supplied by secondary emission in a tube of the type described.

When the light which has reflected from the record subject carried upon the drum II reaches the photo-cathode I3 of the secondary emission photo-tube, it causes emission of electrons from the surface I8 and these electrons flow to the dynode element because of its high direct current potential relative to the cathode. The dynode element 2B which is of relatively large area is formed of a material which readily produces secondary emission and the secondary electrons released from the dyn-ode element 23 are then collected by the screen or collector electrode I9 which is at a still higher positive potential than is the dynode element 23. Thus, due to the release of secondary electrons from the surface of the dynode element 2t, the potential of the dynode elem-ent for increased light val-ues causing release -of a large number of Iphoto-electrons from the surface I8 will become more positive.

In the circuit, as shown, the tone carrier is supplied as `a potential on the photo-cathode I8 of `the secondary emission photo-tube II by way of Vthe transformer 25 :and this causes a modulation ofthe stream of electrons caused by photo-emission from the element I8. Consequently, the output dynode current is a pulsating direct current whose magnitude is proportional to the value of the incident light. However; it can be seen,

`that the output may, under unbalanced conditions, also contain an A. C. component which is the electrostatic leakage current across the interelectrode capacity of the cathode element I8 to the dynode element 23.

In all facsimile systems or other types of systems used to translate light into electric currents the total quantity of light available is usuallyV former 26 serve to provide this counter-electrom-otive force through the resistor capacity network which produces a voltage across the resistor combination 31, 38 which is of proper phase and may be adjusted to proper magnitude to counterlbalance the electrostatically induced voltage upon the dynode l20. The two potentiometers 35 and 36 and the two rheostats 3l and 38 provide this adjustment which is essentially rather critical. After the adjustment is made it is practically fixed for any frequency supplied to the terminal points 2| and for any secondary emission type photo-tube I'l. Under this condition the input energy supplied to the amplifier 46 consists only of the desired modulation voltage developed across the resistor 32 :and some harmonic voltages of the carrier when the light from the record subject ycarried upon therecord drum Il modulates the photo-cathode I'I. The amplifier should preferably be one of relatively high gain since the secondary emission photo-tube output is relatively weak because of the small amount of light available and because of the relatively high speeds With which :the record subject carried upon the drum il is scanned.

Also, the bandpass lter I3 is desirable in order to remove the harmonics of the tone carrier from the output, since these harmonics, especially Ithe second, are usually very large in magnitude in the amplifier output. These usually are present because of the discriminating effect of the counterelectromotive force circuit which does not completely balance them out.

From what has been above stated, it is, of course, to be understood that instead of providing two separate transformers 25 and 26 in the input there may be substituted a single three winding transformer in which the rst winding would be connected to the tone carrier input as shown at the terminal points 2l and the second winding might then be arranged to supply the tone carrier to the secondary emission photo-tube cathode circuit, and the third Winding could supply the tone carrier to the counter-electromotive force circuit. Likewise, it is obvious that the form of amplifier used and shown by the drawing is merely illustrated :as being one which is suitable for use in connection with the system of the type described but obviously any conventionally high impedance input, high gain amplifier, can be used and replace that which has been illustrated.

Similarly, in connection with the counterelectromotive force circuit it is possible to supply the output voltage in series with the resistor 50, for example, or also in series with the resistor 62 instead of in series with the resistor 32 as described and, therefore, it is intended that the foregoing description shall be considered in an illustrative and not in a limiting sense.

Many and other'modifications of this invention can be made without departing from the spirit and scope of the invention in its present form and, therefore, I believe myself able to make all such modifications as are obvious to those skilled in the art which fall fairly within the spirit and scope of this invention.

What I claim is:

l. A transmission system comprising a light converting device including a photoelectric element, a secondary electron emission output electrode and an intermediate accelerating and collecting electrode, a source of carrier frequency energy, a source of light energy, means for energizing the photoelectric element by said light energy, means for applying the carrier frequency energy between the photoelectric element and a point of fixed potential, means for maintaining said output electrode and saidintermediate electrode positive with respect to said point of fixed potential whereby the vcarrier frequency energy present at the output electrode is modulated by secondary electron emission in accordance with the intensity of the energizing light energy, a load circuit connected to receive the modulated output, aV compensating network including the output electrode and the collecting electrode of said light converting device, means for supplying to the compensating network a predetermined amplitude of umnodulated carrier frequency energy, and means for controlling the phase and amplitude of the compensating energy supp-lied to the compensating network so as to neutralize the capacity current flowing between the photoelectric element and the secondary emission output electrode of the light converting device whereby the output of the device to the load circuit may be reduced to substantially zero when no light energy is projected on the photoelectric element.

2'. A transmission system comprising a light converting device including a photo-electric element, a secondary electron emissive output electrode and an intermediate accelerating and collecting electrode, means for applying operating potentials to said output and collecting electrodes with respect to a point of xed potential, a source of carrier frequency energy, a source of light energy, means for energizing the photo-electric element by said light energy, means for applying thefcarrier frequency energy between the photoelectric element and said point of xed potential whereby the carrier frequency energy present at the output electrode is modulated by the secondary electron emission from the output electrode as determined by the photo electrons released in accordance with the intensity of the energizing light, a load circuit connected to said output electrode to receive the modulated output, a compensating network including the output electrode and the collecting electrode of the light converting device, means for supplying to the compensating network a predetermined amplitude of modulated carrier frequency energy, and means for controlling the phase and amplitude of the compensating energy supplied to the compensating network so as to neutralize the capacity current flowing between the photo-electric element and the secondary emission output electrode of the light converting device whereby the output of the device to the load circuit may be reduced to substantially zero when no light energy is projected on the photo-electric element.

WARREN H. BLSS 

